Study On Control And Protection Of Flexible DC Grid Based On PWM Current Source Converter

With the rapid development of power system,high voltage direct current(HVDC)technology has gradually emerged for its long-distance and high-power transmission capability.The flexible dc transmission technology has been well known for its no commutation failure,flexible control mode,no reactive power compensation equipment,and easy to build multi-terminal HVDC power grid.Therefore,flexible dc grid has received widespread attention from all over the world and has become the first choice for long-distance transmission.Nowadays,one of the major challenges to develop the flexible dc grid,such as two-level,three-level VSC or half-bridge sub-module MMC based flexible dc grid,is the effective and economical dc fault protection technique,which is normally realized by the dc circuit breaker(DCCB).However,the requirement of DCCB is very high and the DCCB is quite complex and expensive.Therefore,to explore an alternative solution,this paper proposes a PWM current source converter for flexible dc grid,which can easily limit the dc fault current and reduce the requirement of DCCB.This paper first analyzes the topology of H7-CSC with its operational principle and modulation,which is the improved PWM current source converter(PWM-CSC).The H7-CSC can guarantee the zero current switching ability for the rear-end CSC circuitry to decrease cost.Furthermore,this paper proposes a PWM-CSC with LCL filter added at dc side for constructing a novel flexible dc grid with simple dc fault ride-through capability,where the H7-CSC submodules employ the bidirectional switches for ensuring the mandatory bidirectional power flow.Besides,when the pole-to-ground fault occurs,the bipolar connection mode can ensure continuous and stable operation of the proposed flexible dc grid.In addition,the key electronic parameters design principle of the proposed flexible dc grid is carefully considered.In specific,the operational principle and control scheme of the proposed flexible dc grid are analyzed.Afterwards,the superiority of the proposed flexible dc grid in handling dc faults is analyzed.The detailed dc fault behavior of proposed flexible dc grid using the Laplace transform method is presented.The rising rate of dc fault current is very high when not assuming any protection scheme according to the dc fault transient characteristics analysis,which seriously affect the normal operation of the flexible dc grid.Therefore,the dc fault detection,treatment and recovery method of the PWM-CSC flexible dc grid is proposed.The proposed protection scheme mainly focuses on the cooperative control method for DCCB and converter,which can reduce the design requirement of DCCB for different types of dc faults,even handle dc faults only with mechanical switch.Finally,the proposed flexible dc grid simulation system is set up in PSCAD and a small-scale proposed flexible dc grid prototype is constructed.The PSCAD simulation and experiment results are demonstrated to validate the feasibility of the proposed PWM-CSC flexible dc grid with its control scheme and dc fault protection scheme.